The present invention relates to semiconductor devices including oscillation circuits, and particularly relates to an oscillation circuit which can cope with changes in a driving voltage for a semiconductor device.
In a conventional semiconductor device, for example, reference voltage generating circuits for generating voltages different from each other as reference voltages respectively are provided, and one of the reference voltages is selected in accordance with a command issued from a CPU so that the selected reference voltage is used as a driving voltage for the CPU. More specifically, one of a low driving voltage and a high driving voltage is selected, according to demand, corresponding to the speed of operational processing of the CPU.
However, since the same driving voltage as that for the CPU is applied to an oscillation circuit, the driving ability of an oscillation invertor constituting the oscillation circuit fluctuates depending on the value of the driving voltage as follows.
a) In the case where the driving ability of the oscillation invertor is designed so as to be somewhat low, when the supply voltage for the oscillation circuit is switched to a low value, oscillation is apt to stop, while oscillation hardly starts from the state of stoppage of the oscillation.
b) In the case where the driving ability of the oscillation circuit is designed so as to be somewhat high, when the supply voltage of the oscillation circuit is switched to a high value, high harmonics oscillation occurs so that the device operates erroneously.